This invention relates to a system for sensing oxygen, e.g., which can be used in mixture air-to-fuel ratio control for an internal combustion engine.
It is known to regulate the air-to-fuel ratio of an air/fuel mixture at and around an optimal level by means of a closed-loop or feed-back control system in an internal combustion engine. This closed-loop control system has an oxygen sensor for sensing the oxygen concentration in exhaust gases which represents the mixture air-to-fuel ratio.
Oxygen sensors using titanium dioxide as sensing elements have been recently developed. The internal impedance or resistance of this titania sensor varies as a function of the oxygen concentration in ambient gas.
The internal impedance or resistance of the titania sensor also depends on the temperature thereof. Specifically, the impedance or resistance increases exponentially with decreases in temperature. Accordingly, the titania sensor needs temperature compensation for reliability. When the temperature is lower than a certain value, however, the accuracy or reliability of the titania sensor remains unacceptable regardless of the temperature compensation, since the impedance or resistance thereof is excessively high. The accuracy or reliability remains acceptable at temperatures above that certain value.
In the case where such a titania sensor is used in the closed-loop engine mixture control system, it is desirable to disable or suspend the closed-loop control and regulate the air-to-fuel ratio in another way, such as an open-loop control, when the temperature of the sensor remains within an inoperative or inactive range in which its accuracy or reliability is unacceptable. Such a function necessitates a determination of whether or not the temperature of the sensor is within the inoperative range. Proposed control circuits for effecting this function including the temperature determination have been unsatisfactory, since they tend to degrade the accuracy or reliability of the titania sensor especially at temperatures slightly above the inoperative range.